Sodium sulfide is a solid flake with high alkalinity. It rapidly produces hydrogen sulfide gas (rotten egg gas) when exposed to air, which is highly toxic to the respiratory system. It is also hygroscopic; when it absorbs water, it reacts with water, or when diluted with water. If near flammable substances, it can generate enough heat to cause combustion.
Sodium sulfide comes in various colors depending on concentration and impurities, ranging from red, orange, to yellow. The most commonly used in industry is yellow sodium sulfide, also known as yellow dye rock/yellow flake dye rock in wastewater treatment.